This invention relates to a spark plug, and more particularly relates to a spark plug for spark-ignition internal combustion engines which is improved so as to decrease the undesirable tendency for carbon to adhere to the insulator of the plug.
A spark plug as generally used in spark-ignition internal combustion engines has: a rod-like central electrode which extends along the axis of the plug; an insulator formed of a ceramic material or the like which covers the axial electrode over almost all its length, only the tip of the axial electrode being exposed, and which is generally symmetrical about the axial electrode; and a housing, which is again formed in the general shape of a hollow cylinder, and is made of an electrically conducting material such as metal, which surrounds the insulator with a tubular gap being left therebetween, and which also, at its tip portion, provides at least one ground electrode which opposes the tip of the axial electrode, which is the part of it which is not covered by the insulator. During the use of such a spark plug, inevitably gradually carbon particles adhere to the surface of the insulator which extends between the tip of the axial electrode and the root portion of the plug, where the insulator abuts the grounded housing. Eventually, as they accumulate, these carbon particles lower the surface electrical resistance of the insulator, and in the worst case this resistance drops to such an extent that the plug will not spark, between the electrodes, and engine misfiring occurs, with consequent waste of fuel and poor operating performance of the engine, and also quite possibly increased emission of harmful components in the exhaust of the engine.
Therefore in prior art plugs it has been practiced to make the length of the insulator, extending from the tip to the root thereof, as long as possible, in order to make the resistance of this path, even when lowered by carbon contamination, as high as possible. This surface of the insulator is therefore formed as a long conical or cylindrical surface, and hence, as explained above, a tubular gap is formed between the housing and the insulator.
However, the formation of this tubular gap, in another way, may encourage the formation of carbon deposits. During operation of the engine, air-fuel mixture, burnt air/fuel mixture, sometimes over-rich air/fuel mixture which contains a fine mist of suspended fuel particles, and also soot-laden burnt air/fuel mixture which is the result of combustion of the above over-rich mist-loaded air/fuel mixture, may enter into the deep recesses of this gap, and thereby carbon particles, when produced, are often easily enabled to settle out on the insulator and to adhere thereto. Even though the length of the surface of the insulator between the axial electrode and the ground electrode is quite long, therefore, this kind of plug is not yet free from the problems associated with sooting-up and carbonization.
In view of these problems, it has been proposed to form a constriction on the inner surface of the housing which approaches the insulator at a portion near its tip portion, with an annular gap being left therebetween. In this structure, if the cross-sectional area of the passage between the insulator and the housing in the vicinity of the constriction is made smaller, the circulation of combustible gases into the inner recesses of said tubular gap is more restricted, and the formation of carbon deposits is more effectively reduced. However, it will be apparent that, in order to reduce this cross-sectional area of the passage between the insulator and the housing in the vicinity of the constriction, the width of this annular gap cannot be reduced too much, or otherwise a danger will exist that sparking may occur between the constriction and the axial electrode, which will cause misfiring of the plug, with the attendant disadvantages set out above. Therefore, in general, this width of the annular gap between the constriction and the insulator may not be reduced, with advantage, below a certain minimum width. If then this minimum width is maintained for the annular gap, it will be apparent that, in order to reduce as much as possible the said cross-sectional area of the passage between the insulator and the housing in the vicinity of the constriction, it is advantageous to reduce as much as possible the radius of the insulator at that portion. However, as is well known to one skilled in the art, the heat resistance of a plug is lowered if the insulator is too thin, and the insulator's resistance to cracking is unacceptably low.